This invention relates to a semiconductor light emitting element. More specifically, the invention relates to a semiconductor light emitting element made of InGaAlP materials on an n-type GaAs substrate and including a transparent electrode on its emission surface, which is reduced in operative voltage and increased in optical output by inserting one or both of a carbon-doped p-type GaAlAs layer and p-type GaAs layer between the transparent electrode and a cladding layer.
Semiconductor light emitting elements are widely expanding their field of application to indoor/outdoor displays, railway/traffic signals, compartment/cabin lamps, and so on, because of a number of advantage they have, such as compactness, low power consumption, reliability, for example. Especially, those using as the light emitting layer an InGaAlP material, which is a quaternary compound semiconductor, can be adjusted in composition to emit light in wide wavelength bands from red to green.
In the present application, xe2x80x9cInGaAlPxe2x80x9d pertains to semiconductors of any composition made by changing mole fractions x and y in the composition formula InxGayAl1xe2x88x92xxe2x88x92yP within the range satisfying 0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, and (x+y)xe2x89xa61. That is, mixed crystals such as InGaP, InAlP, InGaAlP, GaP and GaAlP are also grouped into xe2x80x9cInGaAlPxe2x80x9d. Additionally, there are also involved mixed crystals containing arsenic (As) in addition to phosphorus (P) as group V elements.
For years, n-type GaAs substrates using silicon (Si) as the impurity have been typically used as substrates of InGaAlP light emitting diodes (LED).
FIGS. 10 through 12 are cross-sectional views schematically showing InGaAlP semiconductor light emitting elements as comparative examples, which were experimentally made by the Inventor in the course of researches toward the present invention.
The light emitting element 100A shown in FIG. 10 includes an n-type GaAs buffer layer 102, n-type InGaAlP cladding layer 103, InGaAlP active layer 104, p-type InGaAlP cladding layer 105, and ITO (indium tin oxide) transparent electrode 106 sequentially stacked on an n-type GaAs substrate 101, and further formed are a p-side electrode 107 and an n-side electrode 108. The semiconductor layers 101 through 105 are epitaxially grown by metal organic chemical vapor deposition (MOCVD), for example.
In the light emitting element 100B shown in FIG. 11, a p-type GaAlAs current diffusion layer 109 on the p-type cladding layer 105 so that a current injected from the p-side electrode 107 disperses and spreads in a direction parallel to the element surface. The same components as those of the light emitting element shown in FIG. 10 are labeled with common reference numerals, and their explanation is omitted here.
In the light emitting element 100C shown in FIG. 12, a p-type GaAs low-resistance contact layer 110 and the transparent electrode 106 are stacked on the p-type cladding layer 105.
InGaAlP LEDs shown in FIGS. 10 and 12, however, involve serious problems in their operative characteristics. Problems are particularly serious in the structure shown in FIG. 10, and it has not been brought into practice to date. One of reasons of the problems lies in the use of the transparent electrode 106. The purpose of the transparent electrode 106 is to ensure uniform extension of a current along the emission surface and make a uniform light emitting intensity profile. However, the transparent electrode 106 is an n-type semiconductor like ITO (indium tin oxide), for example, and the cladding layer 105 in contact therewith is a p-type semiconductor. Therefore, when a forward voltage is applied to LED 100A, a reverse-biased state is formed between the transparent electrode 106 and the p-type cladding layer 105. As a result, almost no current flows as shown in FIG. 2 as xe2x80x9cComparative Example (1)xe2x80x9d.
The light emitting element 100C shown in FIG. 12 includes, between the p-type cladding layer 105 and the transparent electrode 106, the p-type GaAs low-resistance contact layer 110 doped with a plenty of zinc (zn) (xcx9c1xc3x971020 cmxe2x88x923). As a result, the contact resistance decreases, and relatively good current-voltage characteristics are obtained as shown in FIG. 2 as xe2x80x9cComparative Example (3)xe2x80x9d.
However, here arises another problem caused by doping of a large amount of Zn. Zinc tends to diffuse under heat or current. The large amount of zinc doped into the p-type GaAs layer 110 diffuses not only during crystal growth but also during operation of the element (when a current is supplied), and deteriorates the quality of the active layer 104 for emission, and adversely affects initial characteristics and lifetime characteristics of the element. As a result, as shown in FIG. 7 as xe2x80x9cComparative Example (3)xe2x80x9d, the element is low in optical output, and rapidly deteriorates toward reducing its lifetime. If the dope amount of zinc is reduced (1xc3x971019 cmxe2x88x923 or less), then the p-type GaAs layer 110 loses its function as the low-resistant contact layer, and the element results in involving the same problem of bad current-voltage characteristics.
In the light emitting element 100B shown in FIG. 11, the p-type GaAlAs current diffusion layer 109 is provided instead of a transparent electrode on the emission surface. In the p-type GaAlAs layer 109, mole fraction of Al is increased to transmit light from the active layer 10, and zinc is doped to decrease the resistance. Since a dope amount of zinc of approximately 1xc3x971018 cmxe2x88x923 is sufficient therefor, the problem malfunction caused by diffusion of zinc, involved in he light emitting element 100C shown in FIG. 12, need not be worried about so much. Also, since the transparent electrode 106 is not used, here is not the problem involved in the light emitting element shown in FIG. 10. That is, the light emitting element 100B shown in FIG. 11 has good current-voltage characteristics as shown in FIG. 2 as xe2x80x9cComparative Example (2)xe2x80x9d.
However, this element also involves the problem that, since specific resistance of the p-type GaAlAs layer 109 is not as low as that of the transparent electrode, the current injected through the electrode 107 does not spread uniformly over the entire emission surface of the element. To solve the problem, it is necessary to increase the thickness of the p-type GaAlAs layer 109 as thick as approximately 4 xcexcm or more so as to reduce the current-spreading resistance. However, the thicker layer requires a longer time for the crystal growth, and invites the problem of a higher manufacturing cost.
As reviewed above, in comparative semiconductor light emitting elements, it was difficult to obtain a sufficiently low resistance at the p-side, various approaches for overcoming it invited various problems, such as deterioration of characteristics caused by diffusion of zinc and an increase of the manufacturing cost, for example.
It is therefore an object of the invention to provide an InGaAlP semiconductor light emitting element operative with a lower voltage and ensuring higher output than conventional elements.
According to the invention, there is provided a semiconductor light emitting element comprising: an emission layer made of InGaAlP generating a light; a p-type contact layer made of a semiconductor doped with carbon as a p-type dopant; and a transparent electrode layer in contact with said p-type contact layer, said light generated at said emission layer being emitted through said transparent electrode.
In the present invention, a contact layer doped with a predetermined amount of carbon may be provided to decrease the contact resistance at the contact with the ITO electrode. Unlike zinc, carbon does not diffusion and does not deteriorate the property of the element.
An intermediate band gap layer having an intermediate band gap between those of the contact layer and the cladding layer may be interposed between the contact layer and the cladding layer to alleviate discontinuity between their valence bands, thereby promote inflow of holes and decrease the element resistance.
The present invention is used in the above-explained modes and performs the following effects.
According to the invention, by using the carbon-doped low-resistance ptype contact layer interposed between the p-type cladding layer and the transparent electrode, the contact resistance at the contact with the transparent electrode made of an n-type semiconductor can be reduced effectively, and the element resistance upon application of a forward voltage to the semiconductor light emitting element can be reduced.
Additionally, the p-type intermediate band gap layer interposed between the p-type cladding layer and the p-type contact layer and having an intermediate band gap between band gaps of these layers alleviates band discontinuity of their valence bands, thereby promotes inflow of holes, and further reduced the element resistance.
Moreover, the invention realizes a semiconductor light emitting element remarkably excellent in lifetime characteristics. That is, the invention uses carbon as the dopant for reducing the element resistance. Carbon does not readily diffuse in semiconductors, and removes the problem of deterioration in element characteristics caused by diffusion of the dopant into the active layer. Thus, the invention ensures remarkably excellent lifetime characteristics as compared with those of conventional light emitting elements using zinc.
Furthermore, the invention realizes a semiconductor light emitting element with a uniform emission intensity profile. That is, in conventional light emitting elements using a p-type GaAlAs current diffusion layer, specific resistance of GaAlAs is not as low as that of the transparent electrode, there was the problem that current does not spread uniformly over the entire emission surface. In contrast, the invention ensures sufficient diffusion of current throughout the area from the proximity of the electrode to the outermost peripheral portions of the light emitting element, and thereby promises a uniform emission intensity profile throughout the entirety of the emission surface of the light emitting element.
Simultaneously, the invention improves optical output of the light emitting element. It promises optical output as large as approximately 1.5 times of optical output of a conventional light emitting element.
Furthermore, as a result of remarkable reduction of the element resistance, temperature characteristics of the light emitting element is improved in the present invention. That is, since the heat generation caused by an element resistance decreases in he present invention, the element can be stabilized in operation under high temperatures. This results in enabling the use of semiconductor light emitting elements under various severe conditions heretofore unacceptable, and largely extending the field of applications of semiconductor light emitting elements.
As described above, the invention can provide an InGaAlP light emitting element reduced in element resistance and excellent in emission characteristics and lifetime characteristics, and its industrial merit is great.